Good Morning, Brendan,
You are right in your assessment of the rarity of the British Ammunition wagon. I was fortunate enough to jump on that wagon at the time they were released and got two. I had wanted a couple more for my "wagon train' but they seemed to dry up rather quickly.
The US toy soldier circuit starts in a couple of months and I attend three or more shows (with George - Minutemen Toy Solders) and some local ones so I will keep an eye out for one for you.
I did see one, about six months ago on (I think) the French eBay site, but the shipping cost to the US made my eyes sting!! :redface2:
Good luck. If one pops up on the US ebay I'll let you know.
--- LaRRy